In a wireless communication system there is always the potential of interfering signals that may corrupt a desired signal. This is especially true in the unlicensed ISM (Industrial, Scientific and Medical) and UNII (Unlicensed National Information Infrastructure) frequency bands. These bands of spectrum are available for anyone to use as long as the transmitting device adheres to a limited set of rules. Thus, the protection offered in most other sections of the RF spectrum to insure limited to no co-channel interferes is not offered in the ISM and UNII bands. Therefore, it is desirable to deploy devices which are robust against co-channel interferers in these bands. This can be accomplished using multi-carrier transmission and especially a form of multi-carrier modulation known as Orthogonal Frequency Division Multiplexing (OFDM). While this modulation method is very spectrally efficient and robust against “very narrow” bandwidth interferers, this is not the case with “narrow” and “medium” bandwidth interferers.
Briefly, OFDM is a special case of Frequency Division Multiplexing (FDM) transmission, where a single data stream is transmitted over a number of lower rate sub-carriers (i.e., multiple, narrowband, carriers are used to transmit information in parallel). OFDM can be viewed as either a modulation technique or a multiplexing technique. All the carriers in an OFDM signal are “mathematically” orthogonal to each other so they do not interfere with each other even though they overlap in frequency. Traditionally, a single carrier has been used to transmit data at a very high data rate, whereas OFDM transmits low speed data on multiple carriers. The result is the same data rate using the same amount of spectrum but OFDM is particularly advantageous in multipath environments and environments with “very narrow” bandwidth interferers.
It should be understood that “very narrow” bandwidth interferers, as described herein, refers to interference signals having a bandwidth which is less than the sub-carrier bandwidth. “Narrow” bandwidth signals are considered wide enough to interfere with approximately 5% of the sub-carriers in the signal. “Medium” bandwidth signals are considered wide enough to interfere with approximately 30% of the sub-carriers in the signal. “Wide” bandwidth signals are considered wide enough to interfere with approximately more than 30% of the sub-carriers in the signal. The above convention is used since OFDM systems vary in the number and bandwidth of sub-carriers. One method to compute the bandwidth of a sub-carrier is to subtract the guard interval time from the symbol time and invert the result. The present invention is particularly suitable where there are at least 10% useable sub-carriers.
OFDM is becoming an increasing popular method for modulation. In this regard, OFDM has been accepted for wireless local area network standards from IEEE 802.11, High Performance Local Area Network type 2 (Hiperlan/2), Mobile Multimedia Access Communication (MMAC) Systems, Digital Audio Broadcasting (DAB), and Digital Video Broadcasting (DVB).
An OFDM symbol is comprised of multiple sub-carriers conveying the data. Each sub-carrier can be modulated using some form of phase and amplitude modulation, including, but not limited to, Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16 level Quadrature Amplitude Modulation (16QAM), etc. A common feature for each of the above-mentioned modulation techniques is that the data point can be represented as a vector with a phase and magnitude on a complex plane. To recover the transmitted data the following steps are taken:                (1) A waveform is sampled.        (2) A Fast Fourier Transform (FFT) is computed to convert the time domain waveform into a frequency domain representation (the FFT size is matched to the number of sub-carriers).        (3) Each sub-carrier is corrected for distortions caused by the channel.        (4) A decision is made as to which constellation point was sent.        (5) Decoded data is transferred for higher level processing.        
It should be understood that modulating a sub-carrier using BPSK results in 1 bit/sub-carrier being communicated, using QPSK 2 bits/sub-carrier are communicated and using 16QAM 4 bits/sub-carrier are communicated.
The above-mentioned interference problem is especially acute due to the unlicensed nature of the ISM bands and UNII bands. RF energy from microwave ovens, RF lighting devices and a variety of wireless local area networks (WLANS) operate in the ISM band. The UNII band is relatively free of interference at the present, but that is expected to change as technology becomes more cost effective for this band. Additional interference can be caused by non-intentional radiators.
The prior art addresses the problem of interference by utilizing one or more of the following techniques: (1) frequency hopping (FH); (2) direct sequence spread spectrum (DSSS); (3) waiting until the interference is gone from the channel; and (4) keep trying to send the data until an error free transmission is received. One drawback to these methods is that they suffer from a significant reduction in data throughput.
The present invention allows an OFDM signal to operate with improved throughput in the presence of “narrow” to “medium” bandwidth interferers which, compared to prior art approaches, would have prevented any information from being exchanged.